This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-154888, filed May 25, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a toroidal type continuously variable transmission used as an automotive transmission, for example, and more specifically, to a cam disc of a loading cam mechanism therein.
Investigation is being made into the use of a half-toroidal type continuously variable transmission, such as the one described in Jpn. UM Appln. KOKAI Publication No. 62-71465, for a variator of an automotive transmission. As shown in FIG. 4, the continuously variable transmission of this type includes an input shaft 101, an input disc 102, an output shaft 103, an output disc 104, an adjustable-tilt displacement shaft 105, power rollers 106 rotatably supported on the displacement shaft 105, and the like. The rollers 106 are sandwiched between the input and output discs 102 and 104.
The respective opposite surfaces of the input and output discs 102 and 104 form concave surfaces or traction surfaces 102a and 104a with an arcuate profile. An outer peripheral surface 106a or spherical convex surface of each power roller 106 is in contact with the traction surfaces 102a and 104a. A loading cam mechanism 107 is located behind the input disc 102. The mechanism 107 includes a cam disc 108 having a first cam surface 109, a second cam surface 110 formed on the back of the input disc 102, and rollers 112. The rollers 112 are held in given positions between the cam surfaces 109 and 110 by means of a retainer 111. The cam surfaces 109 and 110 have their respective recesses P1 and projections P2 that are alternately arranged in the circumferential direction of the discs 108 and 102.
If the cam disc 108 rotates as the input shaft 101 is rotated by means of an engine (not shown), the relative positions of the cam surfaces 109 and 110 shift in the circumferential direction of the cam disc 108. As this is done, the discs 102 and 108 bind each other through the medium of the rollers 112 between the cam surfaces 109 and 110, whereupon torque is transmitted from the cam disc 108 to the input disc 102. When the input disc 102 rotates, its rotation is transmitted to the output disc 104 by means of the power rollers 106.
The transmission gear ratio of the output disc 104 to the input disc 102 can be changed by varying the tilt angles of the power rollers 106. Thus, a desired transmission gear ratio can be obtained by changing the ratio between the turning radius at the point of contact between each power roller 106 and the input disc 102 and the turning radius at the point of contact between each power roller 106 and the output disc 104, depending on the tilt angles of the power rollers 106.
As shown in FIG. 5, the cam disc 108 includes the cam surface 109 and a cylindrical portion 108a formed on the central portion of the disc 108. The cylindrical portion 108a projects toward the input disc 102. A clearance groove 108c for grinding is formed in a corner portion between the cam surface 109 and the cylindrical portion 108a. The groove 108c and the cam surface 109 are worked by cam lathe turning that is described in Jpn. Pat. Appln. KOKAI Publication No. 8-61452, for example. After the clearance groove 108c is formed, the cam surface 109 is milled with given accuracy by means of a milling machine.
If a bending load acts on the cam surface 109, however, stress concentrates on a region near the clearance groove 108c of the cam disc 108. This concentration of stress causes the strength of the cam disc 108 to lower. Further, the aforesaid lathe turning for the formation of the clearance groove 108c is required besides the milling work for finishing the cam surface 109 with high accuracy. Thus, the manufacture requires a lot of processes and entails high cost.
If the clearance groove 108c is formed in the cam disc 108, moreover, the retainer 111 for holding the rollers 112 may possibly be caught in the clearance groove 108c. If the retainer 111 is caught in the groove 108c, the rollers 112 are prevented from behaving normally. Accordingly, the loading cam mechanism 107 ceases to be able to generate a predetermined thrust, so that a gross slip or some other trouble may occur in some cases. If the input torque changes suddenly, in particular, the rollers 112 move on the cam surface 109 in a short time, so that the retainer 111 is easily caught in the clearance groove 108c. 
Accordingly, the object of the present invention is to provide a cam disc of a toroidal type continuously variable transmission designed so that concentration of stress on a corner portion between a disc portion and a cylindrical portion can be eased and that the number of working processes can be reduced.
A cam disc according to the invention comprises a disc portion having a cam surface formed thereon and a cylindrical portion protruding from the cam surface in the central portion of the disc portion, and further comprises a lathe-turned portion formed on a part of the cylindrical portion by lathe turning, a milled surface formed on a side face of the cylindrical portion by milling, and a ground surface including the cam surface and a corner curved surface with an arcuate profile formed on a corner portion between the cylindrical portion and the disc portion by grinding.
According to this invention, no clearance groove, such as the one used in a conventional cam disc, is formed in the corner portion between the disc portion and the cylindrical portion, concentration of stress on the corner portion can be eased, so that the strength of the cam disc can be enhanced. Since the corner portion is worked integrally with the side face of the cylindrical portion and the disc portion during the milling work, working processes can be made fewer than in the case of the conventional cam disc with a clearance groove. Since no clearance groove is formed in the corner portion, moreover, there is no possibility of a retainer being caught in a clearance groove.
The cam disc of this invention may comprise a first corner curved surface with an arcuate profile constituting a part of the milled surface and a second corner curved surface with an arcuate profile constituting a part of the ground surface, formed between the first corner curved surface and the cam surface, and having a radius of curvature not smaller than a given value.
A manufacturing method for a cam disc according to the present invention comprises a lathe turning process for lathe-turning at least an end portion of the cylindrical portion, a milling process for working a side face of the cylindrical portion and pre-finishing the cam surface by means of a milling tool, and further forming a first corner curved surface on a corner portion between the cylindrical portion and the disc portion by means of the milling tool, the milling process directly following the lathe turning process, and a grinding process for grinding the cam surface of the disc portion by means of a grindstone so that the first corner curved surface remains and forming a second corner curved surface between the first corner curved surface and the cam surface by means of the grindstone, the grinding process directly following the milling process.
The milling process is carried out by means of, for example, an end mill having an arcuate portion on the distal end portion thereof for working the first corner curved surface. Further, the grinding process is carried out by means of a grindstone having a straight portion for grinding the cam surface and an arcuate portion for grinding the second corner curved surface.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.